1. Field of the Invention
The present invention relates to a lubricating oil composition.
2. Related Background Art
In the field of lubricating oils, additives such as pour point depressants have conventionally been added to lubricating base oils such as highly refined mineral oils to improve the low temperature viscosity properties of the lubricating oils (for example, see Japanese Unexamined Patent Publication HEI No. 4-36391, Japanese Unexamined Patent Publication HEI No. 4-68082, Japanese Unexamined Patent Publication HEI No. 4-120193). Known methods for production of high viscosity index base oils include methods of purifying lubricating base oils by hydrotreatment/hydroisomerization, for stock oils containing natural or synthetic normal paraffins (for example, see Japanese Unexamined Patent Publication No. 2005-154760, Japanese Patent Public Inspection No. 2006-502298 and Japanese Patent Public Inspection No. 2002-503754).
The properties examined when evaluating the low temperature viscosity properties of lubricating base oils and lubricating oils are generally the pour point, cloud point and freezing point. Methods are also known for evaluating the low temperature viscosity property based on the normal paraffin or isoparaffin content of the lubricating base oil.
Lubricating oils are commonly used to ensure smooth operation of machines with sliding section parts, including gears or driven devices such as internal combustion engines, automatic transmissions, dampers, power steering and the like. In particular, lubricating oils for internal combustion engines provide functions of lubrication for parts such as piston rings and cylinder liners, crankshafts, connecting rod bearings, valve gear mechanisms and the like, as well as internal engine cooling, cleaning and dispersion of products and prevention of rust or corrosion.
Such internal combustion engine lubricating oils must not only exhibit a satisfactory low temperature viscosity property but also performance in many other areas, and in recent years it has been a goal to achieve a better trade-off between increased fuel efficiency and improvements in low ash, low phosphorus, low sulfide and long drain performance for exhaust gas after-treatment devices. Because of the large energy loss at areas of friction where lubricating oils function in internal combustion engines, the lubricating oils are combined with various additives including friction reducers, as indicated in Japanese Examined Patent Publication HEI No. 3-23595, for example, to reduce frictional loss and prevent reduced fuel efficiency.
Conventional strategies for reducing the frictional coefficients of lubricating oils have included adding organic molybdenum compounds such as molybdenum dithiocarbamate or molybdenum dithiophosphate, adding combinations of such organic molybdenum compounds with metallic detergents (for example, see Japanese Examined Patent Publication HEI No. 6-62983) or adding combinations of such organic molybdenum compounds with sulfur-based compounds (for example, see Japanese Examined Patent Publication HEI No. 5-83599).
However, significant amounts of soot produced in pistons contaminate the engine oil in diesel engines and direct injection-type gasoline engines. The soot is surface-active and therefore adsorbs the polar additives in the oil, while also chipping at the coating film formed on the friction surface. It has therefore been impossible to obtain a sufficient friction reducing effect under such severe friction conditions even when using organic molybdenum compounds, considered to exhibit the most superior friction reducing effects, because of damage caused by soot and metal abrasion dust. Little research has been conducted on ameliorating this situation, the proposed solutions being limited to mixing alkali metal borate hydrates to improve the fuel efficiency performance of diesel engines (for example, see Japanese Examined Patent Publication HEI No. 1-48319).
It is also known that lowering the viscosity of lubricating oils is effective as a means of increasing fuel efficiency, and multigrade diesel engine oils obtained by adding viscosity index improvers such as polymethacrylates or ethylene-propylene copolymers to low-viscosity lubricating oils are commonly used. Nevertheless, multigrade diesel engine oils containing only such viscosity index improvers have only slight effects of increasing fuel efficiency, and they have been far from satisfactory. Consequently, it is has been highly desired to develop engine oils that can adequately increase fuel efficiency for diesel engines and direct injection-type gasoline engines.
Incidentally, reduction of NOx and suspended particulate matter (SPM) in diesel engines has become a major issue, and various exhaust gas reduction means are being explored, such as high-pressure injection, exhaust gas recirculation (EGR) systems, oxidation catalysts, diesel particulate filters (DPF) and NOx occlusion-reduction catalysts, for the purpose of reducing exhaust gas from such engines.
It is known, however, that the use of such exhaust gas reduction means, and especially oxidation catalysts, NOx occlusion-reduction catalysts and DPF, shortens the life of the exhaust gas after-treatment device depending on the composition of the engine oil that is used. For example, when using a lubricating oil containing zinc dialkyldithiophosphate (hereinafter, “ZnDTP”) which is effective as an anti-wear agent or antioxidant (peroxide decomposer) the zinc in the ZnDTP forms oxides, phosphates or sulfates during the combustion process, accumulating on the catalyst surface or in the filter and impairing the purification performance of the exhaust gas after-treatment device. It is therefore preferable for lubricating oils for engines with such exhaust gas after-treatment devices to contain either no added ZnDTP, or only very small amounts if used. Moreover, since the aforementioned problems tend to occur more easily when metal oxides or sulfuric acid salts accumulate as ash, it is also preferred for the metallic detergent and sulfur contents to be as low as possible.
In addition, large amounts of soot contaminate lubricating oils in diesel engines, and especially EGR-equipped diesel engines, and such soot can lead to increased abrasion in valve gear systems or poor high-temperature detergency of the pistons. The effects of soot contamination and resulting combustion chamber deposits and valve deposits are concerns in direct injection gasoline engines as well. Consequently, special difficulties are associated with simply reducing ZnDTP, metallic detergent and sulfur contents, and new strategies are necessary to deal with the lower detergency and anti-wear properties that result from their reduction.
A diesel engine oil composition with a sulfuric acid ash content limited to no greater than 0.7% by mass has been proposed as a lubricating oil composition for engines equipped with exhaust gas after-treatment devices (see Japanese Unexamined Patent Publication No. 2000-256690). Also, engine oils containing dispersant viscosity index improvers have been proposed to improve detergency against soot contamination and to improve anti-wear properties (see Japanese Unexamined Patent Publication No. 2001-279287, Japanese Unexamined Patent Publication No. 2004-10799). These proposed solutions, however, do not always provide sufficient high-temperature detergency and base value retention when metallic detergents are reduced, while the high-temperature detergency and anti-wear properties in the presence of soot contamination have not been adequately studied in the context of reduced ZnDTP content. A need therefore exists for further research in the areas of maintaining or improving high levels of high-temperature detergency and base value retention while minimizing friction in the presence of significant soot contamination in cases where ZnDTP content has been reduced.